"So days like this have become a bit of the norm. I'm learning that it's ok to fall into bed and give in to the exhaustion or nausea or aches. I've learned a lot of that thru all the beautiful people who have shared their personal stories with me right here on Instagram.

"Thank you. Btw, it doesn't hurt to have #msbowie comfort me So back to my nap. Thank you for your prayers, well wishes and stories. They help so much. #blessed by #mamarosa."

Shannen Doherty broke down (Image: ET)

And it comes after the Charmed actress revealed her dog sensed she was battling cancer before she was diagnosed by a doctor.

The german shepherd began sniffing around the area where her tumour was located repeatedly, and it only began to make sense once she had been diagnosed.

Shannen's dog Bowie (Image: Instagram)

Speaking in an emotional chat with Entertainment Tonight , she said: "When I got back from surgery, [Bowie] again was kind of right in this area.

"Then when I had my first chemo, she would sniff my entire body up and down. And she was always protective before, but she has become this crazy, protective dog. It's hard to get close to me when she's around."

Dogs were previously found to have 98% reliability rate in sniffing out prostate cancer in men, according to research.

A dog sniffing a urine sample (Image: PA)

The research involved two trained German shepherds sniffing the urine of 900 men - 360 of them had prostate cancer and 540 didn't.

One of the dogs was right in 98.7% of the time while the other was right 97.6% of the time.

The study was carried out by the Department of Urology at the Humanitas Clinical and Research Centre in Milan

Meanwhile, Shannen spoke out in the tell-all chat about the devastating last few months as she bravely battles the disease.

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She said: "I had breast cancer that spread to the lymph nodes, and from one of my surgeries we discovered that some of the cancer cells might have actually gone out of the lymph nodes.

"So for that reason, we are doing chemo, and then after chemo, I'll do radiation."

(Image: ET)

The 45-year-old actress - who had a single mastectomy in May - admitted she is worried about the future because so much is "unknown".

She said: "The unknown is always the scariest part. Is the chemo going to work? Is the radiation going to work? You know, am I going to have to go through this again, or am I going to get secondary cancer?

"Everything else is manageable. Pain is manageable, you know living without a breast is manageable, it's the worry of your future and how your future is going to affect the people that you love."

Shannen documented the moment she shaved her hair off on social media (Image: Instagram)

"It was just shedding and it was driving me crazy. It was just clumps, and I was like, 'Just grab the kitchen scissors.' And my mom's like, 'Wait, wait, wait.' I'm just, like, 'Grab it.' She went and grabbed the kitchen scissors, and put it in a ponytail and she just chopped it off. And it was this cute little bob, but it wasn't enough, you know, it was falling out.

"We did stages. We did a pixie. And then we did a mohawk, which was my favourite look. And then finally, we had to get the shaver thing and just buzz it off."

Shannen's husband, Kurt Iswarienko, wasn't able to be with her while she had her head shaved, but his response made her "feel better" about losing her hair.

She recalled: "He was in Mexico working and he was texting nonstop in the middle of his photoshoot like, 'Give me pictures, and are you ok? I wish I could be there.'

A brave move (Image: Instagram @theshando)

Shannen was surrounded by family and friends as she shaved her head (Image: Instagram @theshando)

"He was so stricken that he wasn't there with me but I sent him pictures and videos and he was like, 'Oh my god. You're hot with no hair.' So you know, everybody kind of made me feel better about it."

The actress admitted her treatment has affected her appearance so she wants to be open about her battle with the disease in the hope it helps others.

She said: "People don't realise that cancer -- yes, it ages you -- but also, you can balloon up from the various meds.

"There are so many different reactions you have and so I just -- I didn't want someone to take a picture of me coming out of the grocery store and be like, 'Oh God, look at her.'

Shannen Doherty last year (Image: Rex)

Beverly Hills 90210 (Image: Rex)

"I wanted to put it out there the way it felt the best for me to put it out there. And also, if I could help one person then it makes me go, 'Oh OK.' It's easier to live with having cancer if I know I'm helping at least one person."